1. Field of the Invention
The present invention relates to an aromatic polycarbonate composition having improved thermal stability, and a method for producing the same. More particularly, the present invention is concerned with an aromatic polycarbonate composition comprising an aromatic polycarbonate and a thermal stabilizer, the aromatic polycarbonate comprising a plurality of aromatic polycarbonate chains, each comprising recurring aromatic carbonate units and having a terminal aryl carbonate group and a terminal hydroxyl group, wherein each recurring aromatic carbonate unit has a divalent aromatic or pyridylene group comprising one or more divalent constituent aromatic or pyridylene groups which are each independently unsubstituted or substituted with at least one substituent, and wherein the plurality of aromatic polycarbonate chains collectively contain at least one divalent constituent aromatic or pyridylene group substituted with at least one methyl group in an amount which is not greater than a specific value in terms of the molar ratio of the methyl group-substituted divalent constituent group or groups relative to all divalent constituent groups in the recurring units of the plurality of aromatic polycarbonate chains, the aromatic polycarbonate composition being substantially free of a chlorine-containing compound. The present invention is also concerned with a method for producing the above-mentioned aromatic polycarbonate composition. The aromatic polycarbonate composition of the present invention has excellent thermal stability at molding. Specifically, the aromatic polycarbonate composition of the present invention is advantageous not only in that it is improved with respect to the suppression of discoloration at high-temperature molding, but also in that when it is continuously injection-molded and the continuous molding is interrupted and then restarted, the occurrence of unfavorable phenomena, such as deposition of black spots (carbonized material waste), and development of brown streaks, uneven color and the like, can be effectively suppressed. Therefore, the aromatic polycarbonate composition of the present invention can be advantageously used in various fields of applications of aromatic polycarbonates, such as the use thereof as injection molding materials.
2. Prior Art
In recent years, polycarbonates have been widely used in various fields as engineering plastics which have excellent heat resistance, impact resistance and transparency.
However, aromatic polycarbonates have poor flowability. Therefore, in molding fields (such as injection molding) in which a molded article of an aromatic polycarbonate is required to have excellent surface finish and high transparency, the molding of an aromatic polycarbonate is generally performed at high temperatures. In addition, in recent years, aromatic polycarbonates are increasingly used as molding materials for precision molding which requires a precise transfer is of the configuration of a mold cavity inner wall to the surface of a molded article. Therefore, an aromatic polycarbonate which is suppressed with respect to the occurrence of discoloration even when the molding of the aromatic polycarbonate is conducted at a temperature as high as 350.degree. C. or more, has been earnestly desired. Further, when a continuous injection molding of an aromatic polycarbonate is conducted, a problem arises such that not only do the molded articles tend to have poor appearance, but mold release characteristics are also poor, so that the continuous injection molding has to be frequently interrupted for cleaning the mold, as compared to a continuous injection molding of other types of resins. Moreover, even when the continuous injection molding is restarted after the cleaning of the mold, a problem arises such that unfavorable phenomena, such as deposition of black spots (carbonized material waste), and development of brown streaks, uneven color and the like, markedly occur in the resultant molded articles, and that these unfavorable phenomena continue to occur, so that a large number of poor molded articles are produced before good molded articles begin to be produced. Furthermore, even after steady production of good molded articles has begun again after the restart of the continuous injection molding, molded articles, actually, still sometimes suffer from unfavorable phenomena, such as deposition of black spots, which are fatal defects for injection-molded articles. Also, it is possible that the above-mentioned problems occurring at the restart of the molding after the interruption for cleaning the mold arise even when the molding is restarted after an interruption of the molding for lunch time or weekend, or after an interruption due to an accident to the molding machine or other troubles in the molding process. Therefore, it has been earnestly desired to solve the above-mentioned problems.
It is considered that, for avoiding the above-mentioned unfavorable phenomena, such as deposition of black spots and the like, it is effective to improve the thermal stability of an aromatic polycarbonate. Therefore, various attempts have conventionally been made to improve the thermal stability of an aromatic polycarbonate. However, conventional proposals have been unable to satisfactorily suppress the occurrence of the above-mentioned unfavorable phenomena. For example, for improving the thermal stability of an aromatic polycarbonate produced by the phosgene process (interfacial process), various types of thermal stabilizers, such as thermal stabilizers comprising a phosphorous triester, an epoxy compound, a hindered phenol or the like, have been proposed. By the use of such a thermal stabilizer, the thermal stability of an aromatic polycarbonate can be improved; however, conventionally, an aromatic polycarbonate composition has not been obtained which has advantages such that the occurrence of discoloration is suppressed even when the molding of the composition is performed at high temperatures, and that the occurrence of unfavorable phenomena, such as deposition of black spots, development of brown streaks, uneven color and the like, is suppressed even when the continuous molding of the composition is interrupted and restarted.
It is known that a polycarbonate produced by the phosgene process contains chlorine atoms in a large amount, which chlorine atoms adversely affect the thermal stability of the polycarbonate. It is also known that a polycarbonate produced by the transesterification process is substantially free of a chlorine atom. Recently, a large number of researches and developments have been made for improving the thermal stability of a polycarbonate produced by the transesterification process, especially the melt process.
For example, there have been proposed various melt process polycarbonate compositions, such as a composition comprising a polycarbonate obtained by the melt process using a catalyst containing an alkali metal and a nitrogen-containing compound, and a thermal stabilizer comprising a phosphorus compound, a phenolic compound or an epoxy compound (see Unexamined Japanese Patent Application Laid-Open Specification No. 4-36346); a composition comprising a polycarbonate obtained by the melt process, and a thermal stabilizer comprising a phosphate compound, a phosphonite compound, or a mixture of a phosphate compound and a hindered phenol thermal stabilizer (see Unexamined Japanese Patent Application Laid-Open Specification Nos. 4-15221, 4-15222 and 4-15223); a composition comprising a polycarbonate obtained by the melt process, and an acid phosphate (see Unexamined Japanese Patent Application Laid-Open Specification No. 5-112706); and a composition comprising a polycarbonate obtained by the melt process using a catalyst containing an alkali metal and a nitrogen-containing compound, and an acidic compound (see Examined Japanese Patent Application Publication No. 6-92529 corresponding to EF-A-O 435 124). However, the above-mentioned techniques cannot satisfactorily suppress the occurrence of unfavorable phenomena, such as deposition of black spots, and development of brown streaks, uneven color and the like, which unfavorable phenomena have conventionally been observed when the continuous molding of an aromatic polycarbonate composition is interrupted and restarted.
Also, for obtaining a polycarbonate having improved thermal stability, there have been proposals in which attention is paid to the amounts of trace substances contained in polycarbonates produced by the melt process. As examples of such proposals, the following prior art documents can be mentioned. Unexamined Japanese Patent Application Laid-Open Specification No. 5-148355 discloses a melt process polycarbonate containing, as trace substances, not more than 5 ppm of iron, not more than 1 ppm of sodium and less than 10 ppm of chlorine, and having a terminal hydroxyl group in an amount of not more than 20 mole %, based On the moles of all terminal groups. Unexamined Japanese Patent Application Laid-Open Specification No. 5-156000 discloses a melt process polycarbonate containing, as trace substances, not more than 1,000 ppm of phenol, not more than 1,000 ppm of a divalent hydroxy compound and not more than 2,000 ppm of a carbonic diester. Even by the above-mentioned techniques, however, it has not yet been achieved to obtain an aromatic polycarbonate composition which has advantages such that the occurrence of discoloration is suppressed even when the molding of the composition is performed at high temperatures, and that the occurrence of unfavorable phenomena, such as deposition of black spots, development of brown streaks, uneven color and the like, is suppressed even when the continuous molding of the composition is interrupted and restarted.
The task of the present invention is to provide an aromatic polycarbonate composition which is advantageous not only in that it is improved with respect to the suppression of discoloration at high-temperature molding, especially a molding performed at a temperature as high as 350.degree. C. or more, but also in that when it is continuously injection-molded and the continuous molding of the composition is interrupted and restarted, the occurrence of unfavorable phenomena, such as deposition of black spots, and development of brown streaks, uneven color and the like, can be suppressed. As described hereinabove, such an excellent aromatic polycarbonate composition has conventionally not been known.